The present invention relates to variable gain amplifiers, and more particularly to a variable gain amplifier used in communications devices.
In a wireless communications environment, a wireless communications receiver may receive a signal which experiences rapid and wide variations in signal power. In receivers such as are used in wideband digital code division multiple access (CDMA) mobile stations, it Is necessary to control the power of the demodulation signal for proper signal processing. Also, in transmitters such those used in a CDMA mobile station, it is necessary to control the transmit power in order to avoid excessive interference with other mobile stations. The same power control considerations apply to narrow band analog frequency modulation wireless communications system receivers and transmitters.
Mobile communications receivers and transmitters are designed to have a high compression point, low noise injection and low power consumption. These characteristics are important when designing a variable gain amplifier for a communications system in which signals are transmitted and received over a large range of power levels. A receiver should be able to detect information from both a strong signal broadcast by a nearby and powerful transmitter and a weak signal broadcast by a distant and low power transmitter. The extent over which the receiver can detect weak to strong signals is termed its dynamic range. A transmitter should be able to transmit low powered signals to a nearby receiver and high power signals to a distant receiver.
The dynamic range of a receiver is established by its minimum detectible and maximum detectible signal levels. The minimum detectible signal level of a receiver is determined by the receivers noise figure. The minimum transmittal power is set by the transmitters noise figure if the signal level falls near or below the noise floor. A variable gain amplifiers noise figure is in part a function of the noise injection properties and gain of the amplifier.
The maximum detectible signal level of a receiver may be established by the receiver""s intermodulation distortion performance. When multiple signals pass through any device, mixing action between the signal occurs because of the nonlinearities of the device. A variable gain amplifier""s intermodulation distortion performance is in part a function of its linearity and its gain. In general, the lower the receivers gain, the better the intermodulation distortion performance. This performance is in contrast to the noise figure requirements. Thus, the design of a variable gain amplifier for a receiver with a large dynamic range includes trade offs between intermodulation distortion performance and the noise figure.
Mobile receivers are designed to be compact, light weight and have a long operation lifetime. Because battery voltage is proportional to the number of battery cells, the variable gain amplifier must operate at low supply voltages.
A need has thus arisen for a variable gain amplifier with a high dynamic range, good noise figure and intermodulation distortion performance, as well as having the ability to operate at low power consumption. The variation of the gain due to temperature and precess variation must be minimal.
In accordance with the present invention, a variable gain amplifier includes a voltage divider input stage for receiving a signal to be amplified. A first voltage amplifier is connected in cascade with the voltage divider for amplifying the signal to be amplified. The voltage divider input stage includes a variable impedance circuit having a second voltage amplifier. The second voltage amplifier includes an input and an output. An impedance element is connected between the voltage amplifier input and output. The voltage divider input stage is connected in series to a second impedance element. Each of the voltage amplifiers has its gain controlled by a bias current source.